It has been shown in the prior art that desired position isomers such as the ortho isomer may be obtained by adding .pi.-donating compounds such as polynuclear aromatic compounds such as naphthalene and anthracene to a reaction mixture. Likewise, the prior art has also disclosed that when anisole is subjected to an acetoxylation process, the ortho to para ratio is about 2:1 at low conversions of from 5% to 10% and increases to about 4:1 at a 25% conversion of the anisole. The usual prior art systems which were employed in the acetoxylation of aromatic compounds utilized non-emulsion conditions. This type of reaction required a relatively high operating voltage in the range of about 20 volts in order to obtain a reasonable current density. Therefore, the desired products were obtained at a high cost of power per pound of product.
In many instances, it has been found that the desired position isomer comprises the para isomer and therefore it has been discovered that by effecting the electrochemical oxidation of alkoxy-substituted aromatic compounds in the presence of propionic acid and an alkali metal or alkaline earth metal salt thereof and also in the presence of a phase transfer agent, it is possible to obtain the para isomer in an amount greater than that of the ortho isomer, the system being effected in such a manner so that the selectivity to the desired products is increased while the oxidation of the carboxylate is decreased.
This invention relates to a process for the electrochemical oxidation of alkoxy-substituted aromatic compounds. More specifically, the invention is concerned with a process for obtaining improved yields of the desired position isomer during the electrochemical oxidation of alkoxy-substituted aromatic compounds with a correspondingly lower loss of the attacking species.
Certain chemical compounds, and especially those which contain two substituents in a position para to each other, comprise desired reaction products which are useful in the chemical field. For example, hydroxyanisole may be synthesized electrochemically from anisole. The reaction is carried out in an electrochemical cell so that the desired product is obtained at the anode, said reaction involving the anodic oxidation of anisole in the presence of a nucleophile such as acetate ions which lead to acetoxylation in the ortho and para positions. The para isomer of the reaction constitutes a valuable intermediate inasmuch as the acetoxylated product in which the acetoxy substituent is in a para position is an intermediate for the production of p-hydroxyanisole, this compound being the precursor of t-butylhydroxyanisole which is an antioxidant useful in preventing the oxidation of edible fats and oils. In addition to being admixed with these fats and oils it is also used in food packaging, the wrappings for the foods containing this compound. In addition, other position isomers such as the ortho isomer also constitute marketable compounds of importance in the chemical field.
It is therefore an object of the present invention to provide a process for the electrochemical oxidation of alkoxy-substituted aromatic compounds.
A further object of this invention is to provide a method for obtaining improved yields of desired position isomers which result from the electrochemical oxidation of alkoxy-substituted aromatic compounds.
In one aspect an embodiment of this invention resides in a process for the electrochemical oxidation of an alkoxy-substituted aromatic compound, the improvement which comprises effecting said electrochemical oxidation in an electrochemical cell in the presence of propionic acid, an alkali metal or alkaline earth metal salt thereof and a phase transfer agent comprising a symmetrical or asymmetrical tetraalkylnitrogen or phosphonium-based salt containing from 1 to about 20 carbon atoms in the chain, and recovering the resultant acetoxylated alkoxy-substituted aromatic compound.
A specific embodiment of this invention resides in a process for the electrochemical oxidation of an alkoxy-substituted aromatic compound which comprises treating anisole with propionic acid and sodium propionate in the presence of tetrapropylammonium hydroxide in an electrochemical cell utilizing electrical energy conditions which include a voltage in the range of from about 2 to about 20 volts and a current density in the range of from about 20 to about 200 milliamps per square centimeter (mA/cm.sup.2) at ambient temperature and atmospheric pressure and recovering the resultant p-propoxyanisole.
Other objects and embodiments will be found in the following further detailed description of the present invention.
As hereinbefore set forth the present invention is concerned with a process for the electrochemical oxidation of alkoxy-substituted aromatic compounds whereby a desired position isomer, and particularly the para isomer, of a di-substituted compound is obtained. The electrochemical oxidation is effected by treating an alkoxy-substituted aromatic compound of the type hereinafter set forth in greater detail with propionic acid and an alkali metal or alkaline earth metal salt thereof in the presence of a phase transfer agent in an electrochemical cell. By utilizing such a combination of salts such as the acid salt, water, organic solvent and phase transfer agent, it is possible to provide an emulsion medium in which to effect the electrochemical oxidation of the aromatic compound.
By utilizing propionic acid as the attacking nucleophile during the anodic oxidation of the alkoxy-substituted aromatic compound under emulsion conditions, it is possible to effect the reaction under more favorable conditions than can be found when utilizing other acids as the attacking nucleophile. For example, by utilizing propionic acid, it is possible to greatly increase the percentage of current which is utilized in the desired oxidation of the alkoxy-substituted aromatic compound, to increase the selectivity to the desired products as well as suppressing the oxidation of the attacking nucleophile. The latter is important inasmuch as in the event that less propionic acid is attacked and oxidized the more can be recovered and recycled for further use in the reaction. The decrease in the Kolbe oxidation constitutes an essential advance towards a commercial utilization of the process inasmuch as it will enable the process to be effected in a more economical manner. While the use of other acids as the attacking nucleophile may result in the oxidation of a greater percentage of the desired para isomer over the ortho isomer, this advantage may be nullified or negated by the consumption of the attacking nucleophile, thus necessitating the use of a greater amount of the acid during the reaction, with an attendant rise in the cost of the desired product.
Examples of alkoxy-substituted aromatic compounds (also known as alkylaromatic ethers) which will undergo the electrochemical oxidation will include methyl phenyl ether (anisole), ethyl phenyl ether (phenetole), propyl phenyl ether (propoxybenzene), isopropyl phenyl ether (isopropoxybenzene), n-butyl phenyl ether, sec-butyl phenyl ether, t-butyl phenyl ether, n-amyl phenyl ether, isoamyl phenyl ether, the isomeric hexyl, heptyl, octyl, nonyl, decyl, etc., phenyl ethers, etc.
The aforementioned alkoxy-substituted aromatic compounds are treated with propionic acid and, in addition, an alkali metal or alkaline earth metal salt thereof such as sodium propionate, potassium propionate, lithium propionate, cesium propionate, magnesium propionate, calcium propionate, strontium propionate, etc. The alkali metal or alkaline earth metal salt may be added separately or, if so desired, the alkaline salts may be formed in situ by adding an alkaline compound such as sodium hydroxide, potassium hydroxide, lithium hydroxide, calcium hydroxide, magnesium hydroxide, etc., to the reaction medium, thereby converting a portion of the acid which is present to the salt thereof.
In addition to the presence of the propionic acid and the corresponding alkali metal or alkaline earth metal salt thereof, the reaction is also effected in the presence of a phase transfer agent. In the preferred embodiment of the invention, these phase transfer agents will comprise symmetrical or asymmetrical tetraalkylnitrogen-based or phosphorus-based salts in which the alkyl radicals contain from 1 to 20 carbon atoms in the chain. Some specific examples of these phase transfer agents will include tetramethylammonium hydroxide, tetraethylammonium hydroxide, tetrapropylammonium hydroxide, tetrabutylammonium hydroxide, tetrapentylammonium hydroxide, tetrahexylammonium hydroxide, tetranonylammonium hydroxide, tetradecylammonium hydroxide, tetradodecylammonium hydroxide, butyltrimethylammonium hydroxide, hexyltrimethylammonium hydroxide, heptyltrimethylammonium hydroxide, decyltrimethylammonium hydroxide, dodecyltrimethylammonium hydroxide, hexadecyltrimethylammonium hydroxide, eicosyltrimethylammonium hydroxide, diethyldimethylammonium hydroxide, dipropyldimethylammonium hydroxide, dibutyldimethylammonium hydroxide, dihexyldimethylammonium hydroxide, didecyldimethylammonium hydroxide, tributylammonium hydroxide, triheptylmethylammonium hydroxide, trinonylmethylammonium hydroxide, triundecylmethylammonium hydroxide, tripentadecylmethylammonium hydroxide, dibutyldiethylammonium hydroxide, dioctyldiethylammonium hydroxide, the corresponding sulfate, nitrate, chloride and bromide salts, etc.; tetramethylphosphonium hydroxide, tetrapropylphosphonium hydroxide, tetrapentylphosphonium hydroxide, tetranonylphosphonium hydroxide, tetradodecylphosphonium hydroxide, hexyltrimethylphosphonium hydroxide, decyltrimethylphosphonium hydroxide, hexadecyltrimethylphosphonium hydroxide, diethyldimethylphosphonium hydroxide, dibutyldimethylphosphonium hydroxide, didecyldimethylphosphonium hydroxide, triheptylmethylphosphonium hydroxide, triundecylmethylphosphonium hydroxide, dibutyldiethylphosphonium hydroxide, etc., the corresponding sulfate, nitrate, chloride and bromide salts, etc. It is to be understood that the aforementioned phase transfer agents are only representative of the types of agents which may be employed and that the present invention is not necessarily limited thereto.
In addition to utilizing these various phase transfer agents it is also possible, by varying the chain length of the phase transfer agent which is employed in the reaction, to vary the ratio of ortho to para substituents, the product isomer selectivity being influenced by the number of carbon atoms in the alkyl groups. For example, by utilizing alkyl compounds which are relatively short in nature such as tetraethylammonium hydroxide, it is possible to obtain a greater ratio of para to ortho isomers than can be obtained when utilizing tetraalkyl compounds in which the alkyl radical is relatively long in nature, such as tricaprylylmethylammonium hydroxide.
The electrochemical cell in which the electrochemical oxidation of the alkoxy-substituted aromatic compound is effected may be of any variety which is well known in the art. The electrodes which are employed may be formed of any conductive material, the preferred electrodes in the process of this invention comprising a platinum anode and a stainless steel cathode, although it is also contemplated that other materials such as graphite may also be employed. The electrochemical oxidation is effected utilizing an electrical energy which includes a voltage within the range of from about 2 to about 20 volts and a current density in the range of from about 20 to about 500 mA/cm.sup.2. By utilizing a water emulsion which will include the aforementioned phase transfer agent, propionic acid, and alkaline salt thereof, as well as an organic solvent such as dichloromethane, diethyl ether, acetonitrile, etc., it will be possible to utilize a lower voltage and current density thereby reducing the power cost which will be required to effect the electrochemical oxidation.
The aforesaid components of the reaction mixture will generally be present in amounts ranging from about 0.01 to about 0.2 moles of alkoxy-substituted aromatic compound, about 0.01 to about 0.8 moles of propionate, about 0.02 to about 0.4 moles of propionic acid and about 0.015 moles of phase transfer agent per 100 cc of water.
The process of this invention may be effected in any suitable manner and may include both a batch type and continuous type operation. When a batch type operation is employed, an emulsion which will include the alkoxy-substituted aromatic compound such as anisole, the propionic acid, the alkali metal or alkaline earth metal salt thereof, water, the organic solvent and the phase transfer agent are charged to a flask which is provided with an overhead stirrer, reflux condenser and nitrogen purge tube. In addition, the flask is also provided with a bottom exit tube. The solution is then stirred and transferred from the flask to the electrochemical cell in a multi-pass recycle operation where the alkoxy-substituted aromatic compound is subjected to an electrochemical reaction for a predetermined period of time which may range from about 0.5 up to about 10 hours or more in duration, the electrical energy charged to the cell being within the range hereinbefore set forth. Upon completion of the desired residence time, the mixture is withdrawn from the cell and subjected to conventional means of separation which will include decantation, washing, drying, fractional distillation, etc., whereby the desired product is separated from unreacted starting materials, phase transfer agents, water, organic solvent, etc., and recovered.
It is also contemplated within the scope of this invention that the electrochemical oxidation of the alkoxy-substituted aromatic compound may also be effected in a continuous manner of operation. When such as type of operation is used, the aforementioned components of the reaction mixture, namely, the alkoxy-substituted aromatic compound, the propionic acid, its alkali metal or alkaline earth metal salt thereof, water, phase transfer agent and the organic solvent are also continuously charged to an electrochemical cell which is maintained at the proper operating conditions of temperature and pressure, said preferred conditions including ambient temperature and atmospheric pressure. After cycling through the cell and being subjected to an electrical charge for a predetermined period of time, the effluent is continuously withdrawn and subjected to conventional means of separation whereby the desired product is recovered.
The following examples are given to illustrate the process of this invention in which a preferred position isomer, namely, the para isomer, of an alkoxy-substituted aromatic compound which has been subjected to electrochemical oxidation is prepared and recovered. However, it is to be understood that these examples are given merely for purposes of illustration and that the present invention is not necessarily limited thereto.